1. Field of the Invention
The present invention relates to an optical amplifier capable of amplifying signal light with multiple channels of mutually different wavelengths (WDM signal light), a gain control method for the optical amplifier, and a gain control circuit applicable to the optical amplifier.
2. Description of the Related Art
Utilization of a wavelength division multiplexing (WDM) communication system has been promoted aggressively in recent years in response to demands for increases in communication capacities. Meanwhile, in order to enhance reliability and efficiency of a network, optical add-drop multiplexers (OADM), optical cross-connects (OXC) and the like, which are designed to branch or insert part of the WDM signal light transmitted on the network, are being introduced in such a WDM communication system.
The WDM communication system must cope flexibly with variation in the number of signal channels on the network. Accordingly, flexibility to the variation in the number of signal channels on the network is also required in an optical amplifier such as an erbium-doped fiber amplifier (EDFA), which is widely used as a network constituent in the WDM communication system. Moreover, along with the widespread use of the WDM communication system as described above, it is extremely important to offer optical amplifiers to the market at low prices.
Nevertheless, as a method of controlling amplification gain, conventional optical amplifiers have generally adopted a method of calculating gain by use of a logarithmic amplifier, which is disclosed in Japanese Unexamined Patent Publication No. 2000-40847, for example. To be more concrete, automatic gain control (AGC) for calculating the gain by use of the logarithmic amplifier is conducted based on the following principle.
Specifically, gain of an optical amplifier is represented by the following formula (1):
G(dB)=POUT(dBm)xe2x88x92PIN(dBm)xe2x80x83xe2x80x83(1)
Here, G denotes the gain (unit: dB), PIN denotes a logarithmic value (unit: dBm) of inputted optical power, and POUT denotes a logarithmic value (unit: dBm) of outputted optical power.
On an input side of the optical amplifier, an input-side photodetector outputs a voltage proportional to part of the optical power of signal light before amplification, and the voltage outputted from the input-side photodetector is subjected to logarithmic transformation by an input-side logarithmic amplifier (i.e. an output voltage V1 from the input-side logarithmic amplifier is proportional to the logarithmic value of the optical power detected by the input-side photodetector). On the contrary, an output-side photodetector outputs a voltage proportional to part of the optical power of signal light after amplification, and the voltage outputted from the output-side photodetector is also subjected to logarithmic transformation by an output-side logarithmic amplifier (i.e. an output voltage V2 from the output-side logarithmic amplifier is proportional to the logarithmic value of the optical power detected by the output-side photodetector). Thereafter, the gain of the optical amplifier is detected by subtraction of the obtained voltages V1 and V2 with a difference calculator. The detected gain and target gain are compared by use of a comparator, and gain control of the optical amplifier is conducted by adjustment of pumping light power, for example, such that the detected gain and the target gain coincide approximately with each other.
Meanwhile, in a case of performing automatic level control (ALC) of the optical amplifier, the gain of the optical amplifier is detected as described above, and the outputted optical power is calculated by the following formula (2):
POUT(dBm)=G(dB)+PIN(dBm)xe2x80x83xe2x80x83(2)
Thereafter, the outputted optical power thus obtained and the target outputted optical power are compared, and the pumping light power or the like is controlled such that the above-mentioned power values coincide with each other.
After the studies of the above-described prior art, the inventor of the present invention has found out the following problems.
In the gain control of the conventional optical amplifier, the voltage outputted from the photodetector is subjected to logarithmic transformation with the logarithmic amplifier, and then the gain detection is performed by use of the difference calculator which is readily feasible. However, the logarithmic amplifier is relatively expensive among electronic components. Moreover, a control circuit including the logarithmic amplifier constitutes a nonlinear control circuit which is difficult to design. For example, when the logarithmic amplifier is used, gain of the control circuit may fluctuate owing to the inputted optical power to be detected (i.e. the gain of the control circuit is increased as the power of the light inputted to the photodetector is reduced). Accordingly, the control circuit may become unstable upon an attempt to speed up in such a case, so that it is difficult to achieve high-speed gain control in a wide dynamic range.
The present invention has been made to solve the foregoing problems. An object of the invention is to provide an optical amplifier capable of achieving high-speed gain control in a wider dynamic range with a simpler constitution (with lower costs), a gain control method for the optical amplifier, and a gain control circuit applicable to the optical amplifier.
An optical amplifier according to the present invention includes an optical amplifier medium, a pumping light source, an input-side coupler (a first coupler) and an output-side coupler (a second coupler) disposed to sandwich the optical amplifier medium, and a gain control circuit. The optical amplifier medium includes an erbium-doped fiber (EDF), for example. The pumping light source supplies pumping light of a predetermined wavelength to the optical amplifier medium. The input-side coupler includes a branch port for separating part of signal light to be inputted to the optical amplifier medium. The output-side coupler includes a branch port for separating part of the signal light amplified in the optical amplifier medium. Moreover, the gain control circuit controls gain of the optical amplifier by use of difference information of power values between the light separated by the input-side coupler and the light separated by the output-side coupler. Here, gain control performed by the gain control circuit includes at least automatic gain control.
The gain control circuit includes an input-side photodetector (a first photodetector), an output-side photodetector (a second photodetector), a comparator (included in a control system), and a drive circuit. The input-side photodetector outputs a voltage having a linear relation with the power of the light separated by the input-side coupler. The output-side photodetector outputs a voltage having a linear relation with the power of the light separated by the output-side coupler. The comparator outputs a difference voltage obtained from the voltages outputted respectively from the input-side photodetector and the output-side photodetector. Moreover, the drive circuit supplies a desired drive current to the pumping light source such as a laser diode in response to the voltage outputted from the comparator.
Particularly, in order to perform the gain control of the optical amplifier, the gain control circuit adjusts an inclination a and an intercept b in a function (Vo=axc2x7Pi+b) for defining the linear relation between inputted optical power Pi and an outputted voltage Vo concerning at least one of the input-side photodetector and the output-side photodetector. Here, in order to perform the gain control only of the WDM signal light targeted for amplification, it is preferable that the adjustment of the inclination and the intercept is performed while considering optical power of noise light (mainly ASE light) contained in outputted light of the optical amplifier. Moreover, in order to avoid dynamic gain variation, it is preferable that response time in this gain control circuit is set into one second or less.
In addition to the above-described constitution, it is preferable that the optical amplifier according to the present invention further includes a temperature sensor for measuring an ambient temperature in environment where the input-side photodetector and the output-side photodetector is located. If the ambient temperature varies, there may be a case in which the linear relation between the inputted optical power and the outputted voltage also varies due to a temperature drift in the gain control circuit. Therefore, the gain variation attributable to the temperature variation can be suppressed effectively by adjusting the inclination or the intercept in the linear relation while monitoring the temperature variation properly (i.e. the voltage supplied from the comparator to the drive circuit is appropriately corrected based on a result of measurement by the temperature sensor).
Furthermore, the optical amplifier according to the present invention may also include a gain equalizer disposed between a signal output terminal of the optical amplifier medium and the output-side coupler. By providing the gain equalizer between the optical amplifier medium and the output-side coupler, wavelength dependence of the light separated by the output-side coupler (an object of detection by the output-side photodetector) is eliminated (the unevenness of the gain among respective signal channels is reduced). Accordingly, it is possible to control the gain of the WDM signal constantly irrespective of a change in the number of signal channels of the WDM signal light introduced to the optical amplifier medium.
In the gain control circuit for performing the gain control of the optical amplifier having the above-described constitution, such as the automatic gain control, with a simpler structure and in high speed (i.e. in the gain control circuit according to the present invention), it is preferable that the input-side photodetector, the output-side photodetector and the comparator respectively include circuit elements as follows. In particular, the input-side photodetector includes a photoelectric conversion element (a first photoelectric conversion element) such as a photodiode for converting the light separated by the input-side coupler into an electric current relevant to the power thereof, and an operational amplifier (a first operational amplifier) for converting the electric current outputted from the photoelectric conversion element into a voltage, the operational amplifier being capable of adjusting an offset of the outputted voltage. The output-side photodetector includes a photoelectric conversion element (a second photoelectric conversion element) such as a photodiode for converting the light separated by the output-side coupler into an electric current relevant to the power thereof, and an operational amplifier (a second operational amplifier) for converting the electric current outputted from the photoelectric conversion element into a voltage, the operational amplifier being capable of adjusting an offset of the outputted voltage. Meanwhile, the comparator includes a differential amplifier which receives the voltages outputted from the operational amplifiers respectively included in the input-side photodetector and the output-side photodetector. The differential amplifier outputs a difference voltage between the outputted voltages respectively of the input-side photodetector and of the output-side photodetector to the drive circuit. The drive circuit supplies the drive current to the pumping light source such as a laser diode in accordance with the voltage (the difference voltage) outputted from the comparator.
Particularly, in the gain control circuit according to the present invention, at least one of the input-side photodetector and the output-side photodetector includes an adjusting mechanism for adjusting at least the offset voltage. In other words, in the gain control circuit, at least one of the input-side photodetector and the output-side photodetector adjusts at least an intercept b out of an inclination a and the intercept b included in a function (Vo=axc2x7Pi+b) for defining a linear relation between inputted optical power Pi and an outputted voltage Vo, in which the intercept b corresponds to the offset voltage.
Here, it is preferable that response time in this gain control circuit is also set into one second or less to avoid dynamic gain variation. Moreover, the above-described offset voltage adjustment is performed while considering the optical power of the noise light (mainly the ASE light) contained in the outputted light of the optical amplifier, in order to perform the gain control only of the WDM signal light targeted for amplification.
In addition to the above-described constitution, it is preferable that the gain control circuit according to the present invention further includes a temperature sensor for measuring an ambient temperature in an environment where at least one of the input-side photodetector and the output-side photodetector is located, in order to effectuate stable gain control against temperature variation.
Next, a gain control method according to the present invention is applied to gain control of an optical amplifier as described above for amplifying WDM signal light transmitted through an optical amplifier medium by supplying pumping light of a predetermined wavelength from a pumping light source.
In the gain control method according to the present invention, an inclination a and an intercept b in a function (Vo=axc2x7Pi+b) for defining a linear relation between inputted optical power Pi and an outputted voltage Vo is firstly adjusted by at least one of an input-side photodetector for receiving part of signal light to be inputted to an optical amplifier medium and an output-side photodetector for receiving part of signal light amplified in the optical amplifier medium. Thereafter, a drive circuit for supplying a drive current to a pumping light source is controlled by use of difference information of voltages respectively outputted from the input-side photodetector and the output-side photodetector.
Here, in the gain control method according to the present invention, the drive circuit is controlled based on the difference information of the voltages respectively outputted from the input-side photodetector and the output-side photodetector and on ambient temperature information of an environment where at least one of the input-side photodetector and the output-side photodetector is located, in order to effectuate stable gain control against temperature variation. Moreover, a voltage to be supplied to the drive circuit is adjusted so that amplification gain in the optical amplifier medium becomes constant. Response time from light detection by the input-side photodetector and the output-side photodetector to adjustment of output at the pumping light source is set into one second or less to avoid dynamic gain variation. More preferably, the adjustment of at least one of the inclination and the intercept is performed so as to eliminate influences of noise light contained in outputted light of the optical amplifier.